This invention relates to the field of liquid filters, of the type commonly known as toilet paper filters.
It has long been known that very fine filtration can be obtained, removing particulate contaminants from liquids, by flowing the liquids along an extended filter path across the surface formed between two adjoining sheets of filtration material. This filter action is in distinction to the more common filter action in which a liquid is forced through a membrane or porous material; in the latter case, proper filtration requires very careful design of the membrane, and the size of the particles successfully filtered is a function of membrane porosity and the uniformity of the membrane.
The usual form of the first above described filters is the axial flow, wound fibrous tissue filters, popularly known as paper towel filters. These filters use the filtration capability inherent in passing a fluid axially along the surface between two tightly adjoining sheets of fibrous tissue. The fluid does not pass through any given sheet and thus the normal manufacturing tolerances inherent in membrane or porous filters are not required. Significant work has been done on the design of such filters, because of the low cost and widespread availability of filter media in the form of commercially available paper towels, or toilet paper rolls.
Such filters are used in by-pass oil filtration, and are capable of significant filtration. However, in such use, the filter body is exposed to hot lubricating oil, and, being paralleled with the lubrication system, is under full system lubrication pressure. These filters are intended to run in unattended service, and usually are outside the metered lubrication system. By-pass filtration works by continuously diverting a portion of the lubrication oil flow through the filter during normal engine operation.
The usual form of this filter is a body section, containing a filter cartridge, which is plumbed to the lubrication oil supply. the body is closed off by a lid, which is removable to permit removal and replacement of the filter element.
The joint between the lid and the body is subject to leakage, which if not prevented, will result in gradual loss of all lubricating oil and subsequent engine failure. This leakage problem is an important reason that the use of such filters is not more widespread. As a result, significant effort has been expended in sealing the lids of such filter media against leakage.
Such filters are shown in, for instance, S. K. Yee, et al, U.S. Pat. No. 3,308,957, the Frantz filter, which is considered typical of the paper towel or toilet paper filter, and U.S. Pat. No. 3,504,803 to Brayman and U.S. Pat. No. 4,017,400 to Schade.
Each such patent discloses an axially wound roll of filter material described generally as being a paper towel or toilet paper roll, the rolls subsisting upon a supporting cardboard tube (for instance, the Yee patent, Item 35). This tube is supported axially within the filter canister.
A lid is clamped onto the top of the filter canister, and an O-ring seal is usually placed between the lid and the body of the filter canister to reduce leaks. In order to insure a tight, leak free seal, the canister is made of a dimensionally stable, accurate material. This has usually required that the body and lid be made of machined metal construction, with attendant high manufacturing costs. Molded plastic containers have been tried, but suitably dimensionally stable materials tend to be restricted to low temperature, low pressure service. Lubricating oil filters must be capable of withstanding high temperatures and high pressures.
The clamps for holding together such lids are of a screw adjusted type, which are tightened about the filter canister by finger tightening of a knob, threaded to advance on the screw. This arrangement gives sufficient mechanical advantage to clamp a high dimensional tolerance lid and canister, such as a metal body.
However, when injection molded plastics are substituted for metal bodies, satisfactory, reliable seals have only been achieved in those plastics, such a polycarbonates, which are capable of being molded to the dimensional accuracies of machined metal. The nature of the injection molding process requires that thermosetting plastics be used; by definition, such plastics change shape with temperature, and, for the temperatures typically encountered in lubricating oil service, such dimensionally accurate plastics as polycarbonates cannot be used. Reinforced or composite high temperature injection molded materials, such as glass filled nylons, are much more sensitive to molding conditions, and often display significant dimensional instabilities in such high temperature, high pressure service.